The Acoustic Cavitation Research In Supercritical Carbon Dioxide

Supercritical fluid has many special physical properties, such as a small coefficient of viscosity and surface tension, but the density is similar with liquid. So it has been widely used in the field of extraction, cleaning, etc., especially in supercritical fluid extraction technology. Compared with other supercritical fluid, the critical temperature of carbon dioxide near room temperature, the critical pressure is not too high, but also because of its non-toxic, odorless, nonflammable, low cost, it makes the carbon dioxide supercritical fluid technology become one of the most commonly used medium.In this paper, we analyzed and given the threshold pressure of supercritical carbon dioxide, which considered the existing liquid cavitation theory and physical parameters of supercritical carbon dioxide, and found the cavitation threshold is similar to the hydrostatic pressure. And a simplified Gilmore equation that can be used to calculate the supercritical fluid cavitation analysis were given by consider that the strong compressibility of supercritical fluid and its physical characteristics. According to the experimental data provided by the National Instituteof Standards and Technology(NIST), the density- pressure relationship and the speed of sound- pressure relationship are fitted to solve the Gilmore equation. The effects of Initial radius, fluid temperature, sound pressure amplitude and the sound frequency on ultrasonic cavitation process are analyzed. The research indicates that the threshold of the supercritical fluid is similar to hydrostatic, and when the ultrasonic sound pressure becomes greater than the supercritical fluid cavitation threshold, the movement of the bubbles under ultrasonic are similar to the movement of conventional liquid, such as water. This theoretical analysis demonstrate the feasibility of ultrasound-induced cavitation in supercritical carbon dioxide.According to the cavitation threshold of the supercritical carbon dioxide, design and made an experimental device of the ultrasonic-induced cavitation of supercritical carbon dioxide. The device can provide more than 10 MPa pressure, and the pressure can be controlled. Through the electric heating mode to control and display the temperature of the carbon dioxide. Based on this experiment platform, we conducted two experiments, including cavitation corrosion and the formation of polymer.The cavitation corrosion experiments showed that under the effect of ultrasound, the surface of the Aluminum foil has obvious corrosion spots. And extend the time ofultrasonic radiation, we can find obvious corrosion pit. But without ultrasonic effectct, the aluminum foil has a smooth surface, no cavitation corrosion characteristics appear. On the same condition, the corrosion of the zinc is more obvious than aluminum foil. In the polymer generated experiment, the polymer was characterized by gel permeationchromatography, There are large molecular weight polymer generated in the CO2-MMA system, which demonstrate the feasibility of ultrasound-induces the situ radical formation in Supercritical fluid. All of this confirmed that use high power ultrasound can induce cavitationoccurs in supercritical carbon dioxide.